This invention relates to xerography in general, and more particularly to corona devices used for charging and discharging imaging surfaces used in a xerographic process.
In xerography, uniform electrostatic charges deposited on the surface of a xerographic plate comprising a photoconductive insulating layer supported by conductive backing layer. Exposure to an optical image selectively dissipates the charge and the light struck areas, thus producing an electrostatic charge pattern in image configuration. An appropriate development process, such as dusting with an electroscopic powder which adheres to the charged areas, renders the latent image visible. This powder may then be transferred to a material such as paper by placing the material in face to face contact with the powder image and applying electrical charge to attract powder to the material surface. This copy may then be rendered permanent by fixing, such as by heat fusing.
In the above-noted electrographic process, electrostatic charging techniques are generally relied upon to accomplish such necessary processing steps as the transfer of an electrostatically formed image from a reusable photoreceptor structure to a transfer member and/or tacking and stripping operations associated with such transfer member.
While many forms of acceptable techniques for electrostatically charging a surface are known, corona discharge techniques have generally been preferred in applications such as those mentioned above because such techniques are particularly well suited to applying an electrostatic charge to a moving surface. In addition, the use of corona discharge techniques allows a selected surface to be rapidly charged to a relatively high potential. Conventional forms of corona generating apparatus are illustrated in U.S. Pat. Nos. 2,836,725 and 2,879,395 and generally comprise one or more wire-like electrodes, known as coronodes, horizontally disposed above the surface to be charged and a shield which may take a plurality of different structural forms, partially disposed about a coronode. In one conventional mode of operation, a high voltage d.c. power supply is connected to the coronode with the suitable polarity for the charging operation which is desired, while a conductive layer associated with the surface to be charged is grounded, as are the other terminal of the power supply and the shield.
In electrostatographic machines employing such corona devices, it is desirable to control the charge delivered within known limits. Thus to properly charge a moving xerographic imaging surface a specific rate of flow of charge to the imaging surface is required to obtain optimum copy quality. However, the output of such corona devices is effected by changes in the ambient conditions in which the device operates. Also, accumulations of toner, dust and chemical growths on the device alter its characteristics.
This invention is directed to an arrangement for compensating for the changes resulting from variations in the ambient conditions under which the corona discharge devices operate.
A control arrangement directed specifically to the problem of controlling the output of a corona device in response to changes in power supply voltage is shown in U.S. Pat. No. 3,122,634.
Another corona device control arrangement is shown in U.S. Pat. No. 3,244,683 in which two corona devices are connected across a common supply so that the amount of corona current leaving one corona device is equal and opposite to that entering the other corona device.